Using this mechanism, antibodies can effectively "tag" the antigen (or a microbe or an infected cell bearing such an antigen) for attack by cells of the immune system, or can neutralize it directly (for example, by blocking a part of a virus that is essential for its ability to invade a host cell).

Antibodies may be borne on the surface of an immune cell, as in a B cell receptor, or they may exist freely by being secreted into the extracellular space. The term antibody generally